I'm very interested on having refractive surgery for my nearsightedness, the problem is that 5 years ago I was told that my corneas were too thin (around 407 and 413 microns), even when no pathology was found on them (i.e., ectasia) Taking into account my diopters, after ablation I will still have a stromal bed above 250 microns on both eyes, using PRK-alike techniques.

As a general rule a healthy cornea will remain stable if at least 250 microns of corneal tissue remains untouched (more is always better), however whether or not laser-assisted vision correction surgery is appropriate for you would depend upon many other factors.

I suppose I must add to those numbers an epithelium thinckness ablation of 50 microns using PRK, LASEK or Epi-Lasik.

I know that with those calculations in theory I mantain a stromal bed greater than 250 microns, but I guess surgeons are afraid because my corneas are so thin. Is there any other possible reason not to use one of these refractive surgery techniques, once a possible ectasia or another pathology is discarded? Is there anyone of these three more recommended for me?

On the numbers alone, you are within the treatable range for PRK, LASEK, or Epi-Lasik. A thin flap Lasik may be okay, but your margin of safety would be reduced. I suspect, however, that there is something else that is not here.

A few educated guesses would be the curvature of your cornea is a bit flat and after surgery there would be an increased risk of vision quality problems. Perhaps your pupils are unusually larger and the doctor is concerned about night vision problems. It may be that your Orbscans indicate the possibility of keratoconus - a disease that thins the cornea and can become unstable after vision correction surgery. You will need to discuss with your doctor the reasons for the elevated concern.